The trans-impedance amplifier or TIA is a key component in high-speed communication (e.g., fiber optic) networks and systems. For example, in a fiber optic system, a TIA will convert a signal current from a photodiode into an output voltage. The photodiode signal current varies with both the incident light (e.g., from the fiber optic link) and aging of the system (e.g., of the photodiode, of the driving laser, etc.). Specifically, photodiode AC signal currents can vary from 20 μAp-p to over 3 mAp-p, a range of 150 times or approximately 43 dB, while the TIA output voltage can be required to maintain a fixed level of a few hundred mV peak-to-peak.
Legacy TIA designs were implemented in communication systems deploying non-return-to-zero or NRZ (e.g., two-level) signaling. In these systems, the TIA can be followed by a limiting amplifier to control output variations, since the bit-error-rate or BER requirements are not impacted by the reduced linearity. However, to accommodate the continual demand for more data bandwidth over such networks and systems, multi-level signaling (e.g., pulse-amplitude modulation or PAM) has been deployed. For example, PAM-16 signaling or modulation refers to 16-level signaling as compared to two-level signaling of NRZ. The use of multi-level signaling in turn demands higher performance from the TIAs in the system. Specifically, the TIA needs to accurately reproduce the multiple levels of the signal with low distortion for the subsequent signal processing stages (e.g., continuous-time linear equalizer, analog-to-digital converter, digital signal processing, etc.).
Systems with multi-level signaling can also require a TIA to have high linearity (e.g., 6 bits to 8 bits), which translates to low total harmonic distortion or THD (e.g., −37 dB to −48 dB, respectively). The optical links can further operate at high data rates (e.g., 28 Gbps to 100 Gbps) and require wide frequency response bandwidths (e.g., 6 GHz to 35 GHz). Additionally, the small input current signal levels can require very low noise levels (e.g., less than 1 μArms referred to the photodiode current). Legacy TIA designs using a cost-effective semiconductor manufacturing process and material (e.g., Si CMOS) have higher distortion (e.g., −34 dB) and higher noise levels (e.g., greater than 3 μArms) than required by multi-level signaling systems. Other legacy TIA designs can exhibit improved performance, but at the cost of requiring a specialized semiconductor manufacturing process and material (e.g., SiGe bipolar).
Techniques are needed address the problem of implementing a low cost TIA that exhibits high linearity, low noise, low power, and wide bandwidth. Specifically, none of the aforementioned legacy approaches achieve the capabilities of the herein-disclosed trans-impedance amplifier with replica gain control, therefore, there is a need for improvements.